Autofocus movie camera having pulsed terminal drive means

ABSTRACT

The operational range of an adjustable focus lens is subdivided into a plurality of focus zones, of finite size, by a lens control system that produces a plurality of discrete signals representative of said focus zones, said control system utilizing bidirectional drive means to position the movable element of said lens to one of said focus zones from any position within said operational range in order to focus an image of a remote object at an image plane. The present invention optimally increases the focusing speed and the focusing repeatability of said control system while equalizing and minimizing the focusing error introduced into said control system resulting from the use of such focus zones with bidirectional drive means, by anticipating the arrival of said movable lens element at the desired focus zone, disabling the lens drive means for a limited period of time prior to the time that said lens element arrives at said desired focus zone, and then having said drive means provide a pulsating driving force to said movable lens element until said movable lens element reaches said desired focus zone.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic focusing system for anadjustable focus lens camera in general, and to such a system havingreversible drive means, in particular.

2. Description of the Prior Art

Control systems for automatically positioning the movable element of anadjustable focus lens to a desired focus position in order to properlyfocus an image of a remote object at the image plane of a photographiccamera in response to a signal representative of the distance to saidremote object, have been disclosed in the prior art. Control systemsthat divide the entire operational range of an adjustable focus lensinto a plurality of discrete focus zones by generating a plurality ofdiscrete signals (one discrete signal for each focus zone) in order toso focus an adjustable focus lens, have also been disclosed in the priorart.

Prior art control systems having reversible drive means are capable ofautomatically moving the movable element of an adjustable focus lens ineither of two directions to a particular focus zone (as described above)and stopping lens element movement as soon as said lens element reachessaid focus zone. If the movable lens element is stopped as soon as itreaches an appropriate focus zone, as determined by a focus zone signalcorresponding to said focus zone, said lens element can be positioned toat least two different focus positions for the same focus zone signal,the particular position being dependent upon the end of the focus zonethat is entered by said movable lens element. This can result in a lenselement positioning differential or focusing error as large as the widthof a discrete focus zone.

One fairly common technique for reducing the aforementioned focusingerror is to reduce the width of each focusing zone by increasing thetotal number of focusing zones that collectively represent the entireadjustable focus lens operational range. While this technique doesreduce focusing error as discussed above, it does so by increasing thecomplexity of the automatic focus control system that positions theadjustable focus lens to the appropriate focus zone. For example,increasing the number of discrete focus zones would normally increasethe total number of binary coded bits in a digital control system thatwould be needed to define the additional focus zones for properautomatic focus control system operation. Another technique that mightbe utilized to reduce focusing error is a ratchet and pawl arrangementwhere a pawl engages and arrests the movement of a ratchet that ismounted on and rotates with the movable element of an adjustable focuslens. While this type of arrangement would reduce focusing errors of thetype described above, such an arrangement often breaks down and isrelatively complex and expensive.

In my U.S. Pat. No. 4,178,087, electrodynamic braking is utilized toreduce said aforementioned focusing error by arresting lens movementwithin a selected portion of the desired focusing zone. While thisarrangement is effective at relatively low focusing speeds, the movablelens element tends to pass through or overshoot the desired focusingzone at relatively high focusing speeds which can result in dampedoscillatory movement of said movable lens element within said desiredfocusing zone.

In my U.S. patent application Ser. No. 277 filed on an even dateherewith, the above-mentioned high speed focusing zone overshootingproblem is resolved by sensing the arrival of the lens at a focus zoneimmediately adjacent the desired focus zone and disabling the lens drivemeans for a limited period of time. If the adjustable focus lens is notfocused to the desired focus zone during said limited period of time,said lens drive means is enabled until said lens is focused to saiddesired focus zone. While this arrangement increases the focusing speedof the adjustable focus lens control system, said arrangement does nothave the capability of being able to repeatably position the adjustablefocus lens to approximately the same relative position within all of thecontrol system focus zones which makes the lens focusing error greaterin some focus zones than it does in other focus zones.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, adigital control system having reversible drive means for focusing theadjustable focus lens of a cine camera is capable of repeatably focusingthe movable element of said adjustable focus lens to approximately thesame position within a desired focus zone, at a relatively high rate ofspeed, without causing said lens element to oscillate during such lensfocusing. In such a control system, the entire operational range of saidadjustable focus lens is divided into plurality of discrete focus zonesby generating a plurality of discrete signals, one such signal for eachsuch focus zone. When the focus zone immediately adjacent the desiredfocus zone is sensed by said control system, the driving force providedby said reversible drive means is disabled for a limited period of time.If the adjustable focus lens is not focused to the desired focus zoneduring said limited period of time, said reversible drive means isenabled such that said drive means provides a pulsating driving force tosaid adjustable focus lens until said lens is focused to said desiredfocus zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, in elevation, of an adjustable focus lens motionpicture camera incorporating the inventive concept of the presentinvention.

FIG. 2 is a block diagram of a preferred embodiment of the relativelyhigh speed digital control system of the present invention.

FIG. 3 is a perspective view of the adjustable focus lens, lens mountand reversible lens drive motor of the motion picture camera depicted inFIG. 1 showing the means for encoding the angular and therefore thefocus position of the movable element of said adjustable focus lens.

FIG. 4A is a front elevational view of the lens mount for the movableelement of the adjustable focus lens depicted in FIG. 3, showing a threebit binary code on a disc projecting from said lens mount, said codedefining eight discrete focus zones of said adjustable focus lens.

FIG. 4B is a detailed view of three of the discrete focus zones depictedin FIG. 4A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and, specifically, to FIG. 1, a schematicdiagram of automatic focusing camera 10 constructed in accordance with apreferred embodiment of the present invention, is depicted. Camera 10comprises a housing 12 having handle 14 projecting from the bottomthereof by which a user holds said camera 10 to photograph subject 16through adjustable focus lens mount 18 which directs scene light toimage plane 20 when shutter mechanism 22 is operated. Diaphragm 24associated with shutter mechanism 22, in conjunction with lens mount 18,establishes the instantaneous amount of light incident on image plane20. The opening of diaphragm 24 is controlled by photometer circuit 26in response to available scene light.

Mounted within said housing 12 is automatic focusing system 28 which,when activated, is responsive to the distance to subject 16 from camera10, and to changes in said distance for adjusting the focus position oflens mount 18 in order to maintain an image of subject 16 in focus atimage plane 20. Switch 30, which is mounted in camera handle 14 and isconnected to an energy source (not shown), controls the operation ofphotometer circuit 26 and automatic focus system 28 in response to aminimum amount of pivotal movement of actuator 32 as the handle 14 isengaged by the heel of a user's hand when holding camera 10 in positionto record a scene. Additionally, switch 30 couples said energy source(battery) to motor run switch 34 to permit scene recording as explainedbelow.

Housing 12 also contains motor 36 which, when energized through motorrun switch 34 by depressing trigger 35, simultaneously operates shutter22 and a film indexing claw (not shown) which intermittently dries film38 past an image recording station located behind shutter 22. Finally, aviewfinder 40 is provided to enable a camera user to frame the scenebeing filmed.

In the operation of camera 10, the user grasps handle 14 and framessubject 16 by means of viewfinder 40. As the user holds handle 14,switch 30 is closed by the movement of actuator 32 thereby poweringphotometer circuit 26 and automatic focus system 28. Photometer circuit26 establishes the proper diaphragm opening in accordance with theamount of light in the scene being photographed while automatic focussystem 28 ultrasonically determines the distance to subject 16 and thenfocuses adjustable focus lens mount 18 such that the lens system in saidlens mount 18 focuses as in-focus image of subject 16 at image plane 20when shutter 22 is activated to the open position. The distance tosubject 16 is determined by measuring the time it takes for anultrasonic burst of energy to travel from autofocus system 28 to subject16 and to be reflected back to said autofocus system 28 from saidsubject 16. Reference numerals 42a and 42b designate sequentialultrasonic bursts of energy being transmitted toward subject 16 andreference numerals 44a and 44b designate the reflection of theseultrasonic bursts of energy from subject 16 toward autofocus system 28.In this particular ultrasonic focusing system, an ultrasonic burst ofenergy is transmitted and an echo of said transmitted burst ofultrasonic energy is received before a subsequent burst of rangedetermining ultrasonic energy is transmitted. The ultrasonic rangefinder portion of autofocus system 28 is described in greater detail inmy copending U.S. patent application Ser. No. 916,114.

As discussed above, autofocus system 28 determines the time intervalbetween the transmission of an ultrasonic burst of energy 42a and thereturn of its echo 44a for the purpose of determining the distance tosubject 16 from camera 10. Having established this distance, system 28,when permitted to do so, moves lens mount 18 toward a focus position inwhich an image of subject 16 will be in focus at focal plane 20 whenshutter 22 is activated. As mentioned briefly above, activation ofshutter 22 is selectively carried out when the user depresses trigger 35thereby closing switch 34 and powering motor 36. Autofocus system 28remains in operation so long as the user maintains his grasp of thehandle 14, and is effective to continuously determine subject range andto cause lens mount 18 to track changes in subject distance both priorto and during filming.

Certain details of automatic focus system 28 are shown in FIG. 2, towhich reference is now made. When switch 34 is closed, power is appliedto the components of autofocus system 28 (FIG. 1) which causes systemcycle programmer 48 to divide-down the high frequency output ofoscillator 50 into a transmit and reset pulse train having the samepulse repetition frequency, but shifted in phase. Transmit pulsesproduced at output 52 are designated XMT. The reset pulses produced atoutput 53 and designated RST, are the same as the XMT pulses, but aredelayed with respect to the XMT pulses by about 100 ms, which is greaterthan the round trip time for sonic energy, under normal conditions oftemperature and pressure, for subjects at a distance of about 24 feetfrom the camera 10 (FIG. 1) which represents the hyperfocal lensposition of the lens system mounted in lens mount 18 (FIG. 1). Thisarrangement will allow any echo from a subject within 24 feet of saidcamera to be received by system 28 in the time interval betweensuccessive RST reset pulses.

Transmit and blanking generator 54, to which the XMT pulses and theoutput of oscillator 50 are applied, operates similar to thecorresponding component in the ultrasonic ranging system disclosed incopending application Ser. No. 840,802, filed Nov. 11, 1977, in the nameof JUERG MUGGLI, which causes transducer 56 to transmit periodicultrasonic bursts of energy, two of which are illustrated at 42a and42b. An echo from a subject due to ultrasonic burst of energy 42a,indicated at 44a, is received by transducer 56 where the echo, in theform of an electrical signal, is routed to receiver amplifier 58 in themanner described in the above-mentioned MUGGLI application. Amplifier 58has a ramp gain characteristic controlled by ramp gain generator 60 toincrease the sensitivity of autofocus system 28 to distant subjects. Theoutput of amplifier 58 is detected by receiver detector 62 to produce anecho pulse 63 such that the time between a transmit pulse and itsassociated echo pulse is proportional to the distance between a subjectand camera 10.

This above-noted time interval is utilized in conjunction with scaledclock 64 to establish a number representative of the desired focusposition for lens mount 18. The output of scaled clock 64 is a train ofpulses whose pulse repetition frequency varies with time in accordancewith the derivative of the lens/subject function of the lens systemassociated with lens mount 18. The output of scaled clock 64 isintegrated by accumulating the pulses produced by said scaled clock, inbinary counter 66: the contents of counter 66 at any instant in timerepresents the integral of the time derivative of the lens/subjectfunction evaluated from the time of transmit pulse XMT to said instantin time. Consequently, the contents of counter 66, when echo pulse 63occurs, is a definite integral of the time derivative of thelens/subject function of the lens system associated with lens mount 18,which is a number representing the desired focus position of said lensmount 18 for a subject whose distance is determined by the time intervalbetween transmit pulse XMT and echo pulse 63.

By means of receiver detector 62, echo pulse 63 triggers parallelentry-shift register 68 causing the shifting of the contents of counter66, at the instant of echo pulse 63, into shift register 68. Shortlyafter echo pulse 63 occurs, reset pulse RST appears at output 53 ofsystem cycle programmer 48 thereby resetting scaled clock 64, binarycounter 66, ramp gain generator 60 and transmit and blanking generator54. The condition of autofocus system 28 is now such that upon thegeneration of the next ultrasonic burst of energy in response totransmit and blanking generator 54 and the next transmit pulse XMT fromsystem cycle programmer 48, the cycle of operation described above willbe repeated so that, upon the generation of the next echo pulse 63, thecontents of counter 66 will again be shifted into register 68. As aconsequence, the number in register 68 repeatedly varies in response tochanges in subject distance at a rate dependent upon the pulserepetition rate of transmit pulse XMT.

For determining the actual position of lens mount 18, lens positiondecoder 70 is provided and is described in detail with respect to FIG.3. Reference is now made to FIG. 3 which shows the preferred form oflens position decoder 70. As shown in FIG. 3, lens mount 18 carryingobjective lens 72 is rotatably mounted on threaded member 74 carried bycamera housing 12 so that rotational movement of lens mount 18 causesaxial displacement of lens 72. Actually, the pitch of member 74 isselected such that considerably less than 360° is required to displacelens 72 from its extreme close-up axial position to its infinity orhyperfocal axial position. In order to rotate lens mount 18, a geartrain 76 is interposed between the motor 78 and the gear teeth carriedby the periphery of the mount for objective lens 72. A slip clutchconnection (not shown) is interposed between motor 78 and lens mount 18to permit overrunning of the motor in the event of a jam or engagementof the lens mount with an axial movement limiting stop at either end oflens mount travel. Projecting from and rotatable with the movableportion of lens mount 18 is encoder disc 80, carrying shaft encodingindicia 82 in the form of binary coded slots that pass completelythrough said disc 80. Encoding indicia 82 are preferably in the form ofa gray code. However, for ease of description, a standard three-bitbinary code is utilized. Operatively associated with indicia 82 arethree photocells 84 and three light sources (not shown). The light pathbetween a light source and its associated photocell is blocked andunblocked by slotted encoder disc 80 as said disc is rotated throughsaid light path. The output of each photocell provides one bit ofinformation on the angular and therefore the axial position of themovable element of lens mount 18. The slots in disc 80 and theirrelationship to photocells 84 are shown in greater detail in FIG. 4A.

FIG. 4A is a front elevational view of the movable element in lens mount18 of the adjustable focus lens depicted in FIG. 3, showing a three-bitbinary code on encoder disc 80 projecting from said lens mount, saidcode defining eight address locations or discrete focus zones of saidadjustable focus lens. The eight focus zones are designated A₁ thru A₈,said focus zones corresponding to the numbers 0 through 7, respectively,in binary code. The A₁ thru A₈ focus zones are shown extending over 160°of movable lens element rotation. However, this range of angularmovement is by design choice and said movement range could extend to360° or substantially less than the 160° shown.

Turning again to FIG. 2, the output of lens position decoder 70 isapplied to lens position register 86 which constitutes means responsiveto the position of said lens mount 18 for generating a numberrepresentative of the actual position of said lens mount. Continuing nowwith the operation of the embodiment of FIG. 2, parallel entry/shiftregister 68 is a first register of autofocus system 28 (FIG. 1) andstores a number representative of the desired focus position for lensmount 18, the contents of this first register varying in response tochanges in subject distance at a rate dependent on the pulse repetitionrate of the transmit pulses as previously discussed. Lens positionregister 86 constitutes a second register of autofocus system 28(FIG. 1) which stores a number representative of the actual position oflens mount 18, the contents of register 86 varying in response tochanges in lens mount 18 position at a rate determined by the rate ofchange of lens mount 18 position. The rate of change of the contents ofregister 86 is thus independent of the rate at which the contents ofregister 68 are updated.

The contents of registers 68 and 86 are compared in magnitude comparator88 to determine, on a continuous basis, which register contains thelarger number. Since each register number is based on the same reference(i.e., the desired focus position and the actual lens position aremeasured from the same reference point), the contents of the registerswill be equal when the actual position of lens mount 18 corresponds tothe desired focus position for said lens mount 18. When the contents ofone register exceeds the other, the actual position of lens mount 18will be displaced from its last focus position by an amount equal to thedifference between the contents of each such register. Whether theactual position of lens mount 18 is on one side or the other of thedesired focus position will depend upon which register contains thelarger number.

Comparator 88 has forward and reverse output terminals 92 and 94,respectively. A signal appears on first terminal 92 only when the numberin first register 68 exceeds the number in second register 86. If thenumbers in the registers are designated A and B, then a signal willappear on terminal 92 when A>B. Ordinarily, a signal will appear onsecond terminal 94 only when the reverse relationship between themagnitudes occurs, namely B>A.

In a manner similar to magnitude comparator 88, the contents ofregisters 68 and 86 are compared in adjacent address sensor 96, on acontinuous basis, to determine when a focus zone immediately adjacentthe desired focus zone has been sensed by photocells 84 (FIGS. 1 and 3).Forward and reverse signals 92 and 94 from the output of magnitudecomparator 88 enable adjacent address sensor 96 to determine which oftwo possible focus zones adjacent a desired focus zone is being sensedby photocells 84. For example, in FIG. 4B, which is a detailed view ofthree of the focus zones depicted in FIG. 4A, if focus zone A₅ was thedesired focus zone, focus zones A₄ and A₆ are two focus zones that areimmediately adjacent said focus zone A₅ and for proper implementation ofthe inventive concept of the present invention, adjacent address sensor96 must know which of the two possible adjacent focus zones is theactual adjacent focus zone and this particular information is providedby forward and reverse drive signals 92 and 94 appearing at the outputof magnitude comparator 88. Adjacent address signal 98 appears at theoutput of adjacent address sensor 96 when photocells 84 sense the properfocus zone adjacent a desired focus zone.

In operation, focus forward signal 92 appearing at the output ofmagnitude comparator 88 will appear at the input to OR gate 100 and atthe input to AND gate 102. If adjacent address sensor 96 does notproduce adjacent address signal 98 at its output, AND gate 102 will besatisfied and a drive forward signal will be sent to forward motorcontrol 104 which will cause lens drive motor 78 and lens mount 18 to bedriven in the forward direction toward a desired focus zone. In additionto satisfying gate 102, focus forward signal 92 also satisfies OR gate100 as well as AND gate 106 because the other required input to AND gate106 is being supplied by conducting NAND gate 108. When AND gate 106 issatisfied, timing capacitor C charges up to the output voltage of ANDgate 106, thereby providing one of the two inputs to NAND gate 108. Whenadjacent address sensor 96 senses the adjacent focus zone and producesadjacent address signal 98 at its output, said signal appears at theinput to NAND gate 108, causing said NAND gate 108 to open because ofcapacitor C being fully charged, which causes AND gate 102 to open,which terminates the drive forward signal to forward motor control 104and the driving force being supplied by motor 78, but allowing inertialforces to continue to move lens amount 18 toward the desired focus zone.When adjacent address signal 98 appears at the input of NAND gate 108,it causes said NAND gate 108 to open which, in turn, causes AND gate 106to open. When AND gate 106 opens, the voltage at its output falls,causing the voltage on capacitor C to fall, which again renders NANDgate 108 and AND gate 102 conductive, which again causes forward motorcontrol 104 to energize motor 78, causing said motor 78 to again drivelens mount 18 toward the desired focus zone. Feeding back the outputvoltage of NAND gate 108 to the input of AND gate 106 causes a train ofvoltage pulses to appear across capacitor C and one input to NAND gate108. These voltages pulses cause NAND gate 108 to be gated on and offwhich, in turn, causes AND gate 102 to be gated on and off. Gating ANDgate 102 on and off in this manner causes forward motor control 104 toenergize motor 78 in accordance with the voltage pulses being supplid toAND gate 102 by NAND gate 108. Once the desired focus zone is reached,focus forward signal 92 is disabled, which opens AND gate 102 andterminates the pulsating motor force being supplied by drive motor 78.The characteristics of the voltage pulses appearing across capacitor Care primarily determined by the resistance value of resistor R and thecapacitance value of capacitor C.

Similarly, focus reverse signal 94 appearing at the output of magnitudecomparator 88 will appear at the input to OR gate 100 and at the inputto AND gate 112. If adjacent address sensor 96 does not produce adjacentaddress signal 98 at its output, AND gate 112 will be satisfied, becauseNAND gate 108 will be conducting, and a drive reverse signal will besent to reverse motor control 114, which will cause lens drive motor 78and lens mount 18 to be driven in the reverse direction toward a desiredfocus zone. In addition to satisfying gate 112, focus reverse signal 94also satisfies OR gate 100 as well as AND gate 106 because the otherrequired input to AND gate 106 is being supplied by conducting NAND gate108. When AND gate 106 is satisfied, timing capacitor C charges up tothe output voltage of AND gate 106, thereby providing one of the twoinputs to NAND gate 108. When adjacent address sensor 96 senses theadjacent focus zone and produces adjacent address signal 98 at itsoutput, said signal appears at the input to NAND gate 108, causing saidNAND gate 108 to open because of capacitor C being fully charged, whichcauses AND gate 112 to open, which terminates the drive reverse signalto reverse motor control 114 and the driving force being supplied bymotor 78, but allowing inertial forces to continue to move lens mount 18toward the desired focus zone. When adjacent address signal 98 appearsat the input of NAND gate 108, it causes said NAND gate 108 to openwhich, in turn, causes AND gate 106 to open. When AND gate 106 opens,the voltage at its outputs falls, causing the voltage on capacitor C tofall, which again renders NAND gate 108 and AND gate 112 conductive,which again causes .[.forward.]. .Iadd.reverse .Iaddend.motor control114 to energize motor 78, causing said motor 78 to again drive lensmount 18 toward the desired focus zone. Feeding back the output voltageof NAND gate 108 to the input of AND gate 106, causes a train of voltagepulses to appear across capacitor C and one input to NAND gate 108.These voltage pulses cause NAND gate 108 to be gated on and off which,in turn, causes AND gate 112 to be gated on and off. Gating AND gate 112on and off in this manner causes .[.forward.]. .Iadd.reverse.Iaddend.motor control 114 to energize motor 78 in accordance with thevoltage pulses being supplied to AND gate 112 by said NAND gate 108.Once the desired focus zone is reached, focus reverse signal 94 isdisabled, which opens AND gate 112 and terminates the pulsating motorforce being supplied by drive motor 78.

By sensing the arrival of the adjustable focus lens at an adjacent focuszone, de-energizing the lens drive means when said adjacent focus zoneis reached and .[.the.]. .Iadd.then .Iaddend.continuously pulsing saiddrive means until said lens reaches the desired focus zone, the focuscontrol system can repeatably focus the adjustable focus lens toapproximately the same relative position within any focus zone at arelatively high rate of speed.

It will be apparent to those skilled in the art from the foregoingdescription of my invention that various improvements and modificationscan be made in it without departing from its true scope. The embodimentsdescribed herein are merely illustrative and should not be viewed as theonly embodiments that might encompass my invention.

What is claimed is:
 1. In an autofocusing camera having a displaceablelens,means for determining subject distance, control means energizeablefor displacing said lens to a location selected in accordance with saiddistance determining means to thereby focus an image of the subject onthe camera focal plane, said control means including encoder meansresponsive to lens displacement for producing a plurality of discretesignals, each representative of displacement of said lens to within arespective positional zone of said lens; and said control meansincluding means for terminating displacement of said lens within apositional zone selected in accordance with said distance determiningmeans, the improvement comprising: that said displacement terminatingmeans includes, means for generating a signal representative of saidlens at the adjacent positional zone immediately before said selectedpositional zone; and means for regulating said control means such thatit provides a constant force to said displaceable lens before it ispositioned to a positional zone immediately adjacent said selectedpositional zone and provides a pulsating force to said displaceable lenswhile said lens is positioned within said adjacent positional zone. 2.The apparatus of claim 1, wherein said pulsating force regulating meansincludes a resistor and capacitor pulse forming network and the outputof said network is fed back into its input.
 3. The apparatus of claim 1,wherein all of the pulses produced by said pulsating force drive meansare of uniform amplitude and width.
 4. The apparatus of claim 1, whereinthe length of time of a force pulse and the time between adjacent forcepulses are equal.
 5. The apparatus of claim 1, wherein said adjacentzone signal generating means includes means for determining the adjacentpositional zone immediately before said selected positional zone.
 6. Ina photographic camera of the type having, means for selectively couplingsaid camera to a source of energy,an image plane, an adjustable focuslens mounted for displacement over a given operational range where itserves to focus images of subjects positioned within a range of subjectdistances at said image plane, means for producing a signal indicativeof the position of a particular subject within said range of subjectdistances, means for producing a plurality of discrete focus zonesignals that correspond to a plurality of focus zones, said focus zonesrepresenting the entire focusing range of said adjustable focus lens,drive means for effecting movement of said lens from either of twodirections to a desired focus zone to focus an image of a particularsubject at said image plane, the improvement comprising: means forgenerating a signal representative of said lens at the adjacent focuszone immediately before said particular focus zone; and means forregulating said drive means such that it provides a constant force tosaid adjustable focus lens before it is positioned to a positional zoneimmediately adjacent said selected positional zone and provides apulsating force to said adjustable focus lens while said lens ispositioned within said adjacent positional zone.
 7. The apparatus ofclaim 6, wherein said pulsating force regulating means includes aresistor and capacitor pulse forming network and the output of saidnetwork is fed back into its input.
 8. The apparatus of claim 6, whereinall of the pulses produced by said pulsating force drive means are ofuniform amplitude and width.
 9. The apparatus of claim 6, wherein thelength of time of a force pulse and the time between adjacent forcepulses are equal.
 10. The apparatus of claim 6, wherein said adjacentzone signal generating means includes means for determining the adjacentpositional zone immediately before said selected positional zone. .Iadd.11. An autofocus camera comprising:means for determining subjectdistance; an adjustable lens displaceable for focusing an image of thesubject on the focal plane of the camera; drive means for displacing thelens; and control means for controlling said drive means to provide arelatively constant driving force to said lens before it reaches aposition in advance of the focal position desired in correlation withthe determined subject distance, and for thereafter providing apulsating driving force to said lens until said lens reaches saiddesired focal position. .Iaddend..Iadd.
 12. The camera of claim 11wherein the control means includes means for disabling said drive meansfor a limited period of time following provision of said constantdriving force, and thereafter providing said pulsating driving forceonly if said lens has not reached said desired zone within said limitedperiod of time. .Iaddend..Iadd.
 13. The camera of claim 12 wherein theoperational range of said lens is divided into a plurality of focuszones, and said control means includes means for controlling said drivemeans to provide said relatively constant driving force to said lensbefore it reaches a zone adjacent the focal zone desired in correlationwith the determined subject distance and for thereafter providing apulsating driving force to said lens until said lens reaches saiddesired focal zone. .Iaddend.